Field Trip 3: San Andreas Fault, Mormon Rocks, and Devil's Punchbowl

Tilted strata of the Punchbowl Formation at Devil's Punchbowl Natural Area.

Overview

This field trip includes stops along the San Andreas fault Links to an external site. (SAF), Mormon Rocks, and Devil’s Punchbowl Natural Area; students can observe and study the effects of faulting and crustal deformation at these locations. It also provides an opportunity to observe and discuss sedimentary rocks and depositional environments and make connections between plate tectonic stresses and plate boundaries.

Google Earth Map the San Gabriel and San Bernardino Mountains being bisected by the San Andreas fault. Can you see it??

Logistics

All stops will be on or very close to the San Andreas fault, which trends across the southern margin San Bernardino Mountains and the Northern margin of the San Gabriel Mountains. 

Devil’s Punchbowl has an attractive picnic area, an intriguing nature center, and several trailheads. Considering weather forecasts, conducting this trip during late fall, winter, or spring is best.

Geology

At 750 miles long, the SAF is California's most significant tectonic structure. This right-lateral strike-slip fault and its subsidiary faults comprise the SAF zone, representing the main trace of the plate boundary between the Pacific and North American plates and accommodating about one inch of plate motion per year. In actuality though, the plate boundary between the Pacific and North American plates is more like a smeared deck of cards than a single fracture of the crust. This analogy works because the main trace of the fault accounts for only about one-third of the movement between the two plates over the past 25 million years. Instead many active and inactive parallel faults, some west of the SAF, like the San Gabriel fault Links to an external site. in the western San Gabriel mountains and others as far inland as western Nevada, make up the difference. 

Relative movement between these plates happens as intermittent slippage along the SAF and associated faults, resulting in major earthquakes, such as the magnitude 7.9 San Francisco earthquake in 1906 and the magnitude 7.9 Fort Tejon earthquake in 1857; the latter being the last "big one" in southern California. Careful analysis of strata offset by past earthquakes shows that major earthquakes happen along the SAF at an average rate of one every 150 years. Major earthquakes typically disturb the ground surface, making ground cracks and fault scarps when one block of crust is uplifted relative to the crust on the other side of the fault. Since 1857 erosion and urban development have made tracing the fault on the Earth’s surface difficult. Instead, we can use topographic and geologic evidence to locate a fault's position, such as stream offsets, sag ponds, eroded scarps, and juxtaposition of distinct rock units. Participants in this field trip can observe some of this evidence firsthand at the first two stops. 

At Mormon Rocks, there are striking examples of how tectonic forces can uplift and tilt strata and opportunities to discuss the formation, texture, and depositional setting for sedimentary rocks. Devil’s Punchbowl provides an excellent opportunity to build upon the earlier field trip stops, being situated just south of the SAF and strata deformed into a plunging syncline. Adding to the geologic appeal, the rock at Devil's Punchbowl is deceptively similar to the lithology of Mormon Rocks. 

Learning Objectives

Through participation in this field trip students should be able to:

1. Name the type and specific name of the plate boundary crossed en route to Devil’s Punchbowl
2. Use stratigraphy and texture to identify sedimentary rocks
3. Identify stream terraces and unconformities
4. Locate faults using indirect evidence, such as the juxtaposition of the rock units or topographic features
5. Describe the geologic significance of the “inface bluffs”
6. Apply the Law of Original Horizontality
7. Identify and draw a syncline
8. Compare and contrast the Cajon and Punchbowl formations 

Key Vocabulary

• Alluvial fan – a fan-shaped pile of sediment that was deposited intermittently by flash floods/debris flows at the mouth of a mountain canyon
• Detrital – texture of sedimentary rock composed of clasts or grains of rocks and minerals
• Law of Original Horizontality – sediment is always deposited as strata that are horizontal or very nearly horizontal
• Plunging syncline – a syncline with a fold axis that is inclined or has a “plunge”
• San Andreas fault – Major right-lateral strike-slip fault and transform plate boundary between the Pacific and North American plates
• Strata – a vertical sequence of layers of sediment and is considered the definitive characteristic of sedimentary rocks
• Stream terrace – topographically flat surface elevated above the active stream channel representing the past position of the stream bed or flood plain for the stream 
• Syncline – a “U”-shaped structure typically caused by the compressing and tilting of strata around an imaginary axis (fold axis)
• Syncline (plunging) – a syncline where the fold axis has been tilted from horizontal, resulting in the tilting of the syncline structure
• Unconformity – contact between older rock below and significantly younger rock above, representing a gap in geologic time caused by prolonged erosion and/or lack of deposition

Pre-Field Trip Questions

1. What characteristic of sedimentary rocks makes them easily recognizable in the field?
2. If inspecting a detrital sedimentary rock what would you see that makes it “detrital”?
3. Examine a few pictures of alluvial fans: Google images of alluvial fans. Links to an external site.
4. Using the illustration below, describe the geologic structure represented by the Punchbowl Formation (abbreviated as "Mp").  
   1. 
5. Watch this short video, The San Andreas Fault Links to an external site., then answer this question, “How likely do you think it is that we could experience the Big One, a major earthquake on the San Andreas fault, while we are on our field trip?” Explain.
6. If a major earthquake happens on the San Andreas fault every 150 years, and the last major earthquake was in 1857, when should the next one happen? Considering your answer, what does this mean?

Directions to Blue Cut (stop 1) :

At the I-15/215 interchange, look for Kenwood Ave (exit 124), accessed via the truck route for I-15 North. Turn left and pass under the freeway. Turn right onto Cajon Boulevard (Route 66). Proceed 3.7 miles to the northeast, then pull over onto the northern shoulder alongside the stone wall.

Field Trip Stops

All stops listed in this chapter are accessible by large charter bus, van, or 2-wheel drive car.

Map showing the approximate locations of the field trip stops.

Stop 1 – San Andreas Fault and the Blue Cut

Addresses learning objectives:
1. Name the type and name of plate boundary crossed en route to Devil’s Punchbowl
4. Identify stream terraces and unconformities
5. Use indirect evidence, such as the juxtaposition of rock units or topographic features to locate faults

Geography

Blue Cut is situated in the Cajon Pass, an erosional valley made by Cajon Creek, containing old Route 66, the BNSF and UP rail lines, and the I-15. It also separates the San Gabriel Mountains from the San Bernardino Mountains, which, facing north, are off to our left and right, respectively.

Geology

This area is named the Blue Cut because of exposures of the Pelona Schist, which conveys a blue-gray hue. This geologically significant rock unit represents marine sediments deposited atop the oceanic Farallon Plate, which was subducted under the North American plate 200-25 million years ago, metamorphosing the sediments into schist (metamorphic rock). The Pelona Schist is a “lower plate” rock unit because it was subducted beneath the North American Plate. Today this significant unit is only sporadically exposed. Uplift along the Punchbowl fault, a splay of the SAF, and erosion have revealed one of these rare “tectonic windows.” into the lower plate. There are other metamorphic rock units here as well, thanks to faulting, making this relatively small area geologically complex.

Across Cajon Creek, note the 8000-year-old stream terrace supporting the train tracks and the active stream channel far below. This amount of downcutting through bedrock is a testament to the rapid tectonic uplift of the San Gabriel Mountains. One can also observe an unconformity (nonconformity) between the 75 million-year-old Pelona Schist and light-colored Quaternary gravels. 

The hill on the opposite side of Cajon Blvd is composed of Pelona Schist, choloritized gneiss, and chloritized tonalite (Hazlett, R. Pomona College field trip guide). Strands of the Punchbowl fault bisect it. Further inspection reveals multiple landslides, as the structurally weak Pelona Schist is susceptible to mass wasting. 

Our current location puts us in the middle of a nearly right-angle turn of Cajon Creek caused by the right-lateral movement of the SAF. Cajon Blvd reflects this bend.

Unconformity between Cretaceous age Pelona Schist (gray) and Quaternary age gravels (pinkish tan). 

Photograph of Pelona Schist in Blue Cut area.  

Note the dramatic bend in Cajon Creek caused by right-lateral movement along the San Andreas fault - Cajon Blvd accentuates this bend

Looking north along old Route 66; the bend in the road reflects the bend of Cajon Creek. 

Student Activity 1: Make a map view drawing showing the path of Cajon Creek before offset from the SAF and a second drawing showing the stream as it is now.

Map view perspective of Cajon Creek with the estimated location of the San Andreas fault. - Google Earth 

The Blue Cut area is situated on the edge of the Pacific Plate. To reach the North American Plate, return to your vehicles and drive about ½ mile northeast. At the end of the hills on the right (those containing the Pelona Schist), you are on the main strand of the SAF zone and the plate boundary for the two plates. 

The rock outcropping in hills off to the left, on the far side of Cajon Creek, are on the North American Plate and is significantly younger (~ 50 million-year-old sedimentary rock) than the rock supporting the hills on the Pacific Plate side of the fault (75 million-year-old Pelona schist). These different rock bodies are juxtaposed due to the SAF moving them to their present positions.

The SAF

The right-lateral strike-slip SAF system is more than 800 miles long and extends to depths of at least 10 miles within the Earth. The SAF and its many subsidiary branches have accommodated approximately 350 miles of plate movement between the Pacific and North American plates  (USGS). 

Student activity 2: Research indicates that this fault segment should be moving about 1 inch per year, but friction has prevented this part of the fault from moving since before 1700. If the fault were to slip today, how many feet would you expect the road on the other side of the fault to shift? Show your work as you convert from inches to feet per year. Start by subtracting 1700 from the current year to get the total inches, then convert inches to years. In what direction would the ground on the other side of the fault move relative to us (right or left)?

Student activity 3: Make a map view drawing of the SAF that shows how Route 66 would be offset by movement along the fault. Include a scale to provide an approximation of the amount of total offset. 

Stop 2 – Mormon Rocks

Addresses learning objectives:
2. Use stratigraphy and texture to identify rocks as sedimentary
3. Distinguish clastic/detrital texture in rock
6. Describe the geologic significance of the “inface bluffs”
7. Describe how the orientation of the strata of Cajon Formation and the Punchbowl Formation violates the Law of Original Horizontality and explain why this has happened
9. Explain why the Cajon and Punchbowl formations are not the same body of rock despite having very similar lithology

Continue on Cajon Blvd towards the I-15. As you drive onto the North American Plate, ask students what type of rock makes up the hills off to the left (~50 mya sandstone and shale).

Take I-15 north to the next exit, CA-138, turn left, and go 1.5 miles to the Mormon Rocks; park on the unpaved turnout immediately after crossing the bridge on the right side (north) of CA-138 and directly across from Lone Pine Canyon Road. 

From the parking area, walk across the desert scrub brush mantled floodplain towards Mormon Rocks, then down into the Cajon Creek wash. From here, take a moment to observe the outcrops of the Cajon Formation or Cajon Valley Formation (Kenney, M.) that make up Mormon Rocks. 

Student activity 4: Ask, “Do the strata making up the Cajon Formation agree with the Law of Original Horizontality?” – No. “Why not?” – It isn’t horizontal. “What happened to the strata since deposition and why?” – Tectonic stresses tilted the beds, so they jut out of the ground.

Walk up to the outcrops.

Student activity 5: Instruct students to make a sketch representative of an approximate 1-foot by 1-foot section of the Cajon Formation so the drawing shows stratification and a few of the clasts or grains. Students should also note the color and texture of the grains because they will be asked to compare these to the pebbles and cobbles contained in the Punchbowl Formation later on this field trip. Students who are more advanced could also be asked to identify the specific rock type that makes up the grains in their drawings.

Ask, “What was the depositional environment where this sediment was deposited?” – energetic streams/braided streams.

Geology

Cajon Formation 

• Sandstone with some conglomerate deposited by braided streams 15-18 million years ago as indicated by horse and camel fossils (Prothero, 2011)
• Tectonic stresses have tilted the strata to about 45o
• Holes in strata could be the result of larger grains being “weathered-out” and subsequent wind erosion enlarging the hole over time OR are from a different weathering process called tafoni:
• Tafoni are common weathering phenomena in semiarid regions (Trent, D., personal communication) where rock outcrops intersect the soil horizon
• Alternating wetting and drying results in chemical weathering of the rock at ground level, along the soil horizon, resulting in holes in the rock
• Erosion of the older soil horizon and subsequent tilting leaves the holes suspended above the current ground level

Ask, "What evidence could support the tafoni hypothesis?" - holes are along the same horizontal plane

Generalized geology of the Cajon Pass area. Map by Dee Trent from “Field Trip 1: Geology Along the San Andreas fault in the Cajon Pass-Wrightwood areas, Southern California,” Hazlett, R.

Return to vehicles and continue west on CA-138 for 3.9 miles, where you’ll pull over on the shoulder of the highway at Mt. Shadow Manor Lane (or anywhere you can safely do so before or after Mt. Shadow Manor Lane). If you feel that you’ll have enough space to safely assemble your group on the side of the bus opposite the road, then you could exit vehicles for a discussion about the inface bluffs (see below). Alternatively, discuss from within your vehicle.

Stop 3: Inface bluffs (optional)

The inface bluffs are a cross-sectional view of the interior of alluvial fans that have been disconnected from the San Gabriel mountain front from which they were receiving their sediment. This happened when the ancestral SAF bisected Cajon Creek.  Streams, like Cajon Creek, erode along a path of least resistance. In this case, that path is along the SAF, where the rock has been ground-up, making it weaker and more easily erodible by movement along the fault. This led to the headward (upstream) erosion of Cajon Creek, expanding the drainage basin and eroding the soft alluvial fan deposits brought into fault contact with Cajon Creek and its tributaries. 

Formation of the Cajon Pass area. Map by Dee Trent from “Field Trip 1: Geology Along the San Andreas fault in the Cajon Pass-Wrightwood areas, Southern California,” Hazlett, R.

Relative to us, the movement of the North American plate has shifted these alluvial fans to our right, with their interiors now exposed see “C” in the illustration below (Sylvester and Gans, 2016).

Diagram of an Alluvial Fan from different perspectives.

Continue west on CA-138 for about 24.5 miles to 131^st/Longview Road (N6); turn left (south) and continue for about 2.2 miles to Fort Tejon Road, where you turn left, then make a quick right onto the next street, 131^st/Longview Road (N6). Proceed about 2.3 miles to Tumbleweed Road (look for signs for Devil’s Punchbowl) and turn left. Drive until the road ends at the parking lot for Devil’s Punchbowl; note: Tumbleweed Road turns into Devil's Punchbowl Road. The parking lot is often full, so expect to park along the road just before the parking lot. If arriving by charter bus, ask the driver to enter the parking lot (the driver should be able to turn the bus around in the parking lot) to unload the group then park where space is available in the parking lot or along the road.

OR 

“Stop” 4: Lone Pine Canyon Links to an external site. 

For this “stop,” we won’t be stopping but will drive along the main trace of the SAF. Extra credit if the Big One happens while we’re on it! 

En route to stop 5, look for evidence of the SAF in the form of fault gouge exposed in roadcuts. 

Stop 5: Jackson Lake

Addresses learning objectives:

• Use topographic features, like scarps or vegetation patterns, to locate the trace of a fault
• Enjoy lunch in a peaceful setting

Water has accumulated on one side of a shutter ridge, the hill on the far side of the lake. This ridge formed by lateral offset along the SAF, which dammed a stream and made a lake behind the shutter ridge (Hough, 2004).

Stop 6 –Devil’s Punchbowl Natural Area  

Before leaving vehicles, remind students to take everything with them, as you’ll be away from the bus for 1.5-2.5 hours. Assuming you start your visit with a lunch break, direct students to the picnic area, about 50 yards beyond the far right corner (southeast) of the parking lot. You may want to point out the port-a-potty in the parking lot and the other restrooms, which are to the left of the visitor’s center. Finally, it’s probably a good idea to designate a meeting place and time for your lecture to prevent students from wandering too far off. The “punchbowl overlook” (see below) is a great spot to discuss the geology that makes this a special place.

Addresses learning objectives:
2. Identify sedimentary rocks and strata
3. Distinguish clastic/detrital texture in rock
7. Describe how the orientation of the strata of the Cajon Formation and the Punchbowl Formation violates the Law of Original Horizontality and explain why this has happened
8. Identify and draw a syncline
9. Explain why the Cajon and Punchbowl formations are not one and the same despite appearing very similar

Punchbowl Overlook

Walk north between the visitor’s center and the building opposite the visitor’s center, with owl cages outside, to find an overlook with excellent views of the plunging syncline that gives this place its name. It’s a good spot to give a lecture to start your visit. Note that you may wish to withhold some of the information below from your students until after they have completed Activity 8, not to give away answers to the suggested questions.

Geology

Punchbowl Formation

• Conglomerate and sandstone beds represent braided river and alluvial fan deposits from 7-10 million years ago (as opposed to 15-18 million years old for the Cajon Formation)
• Grains are generally coarser and less mature than those in the Cajon Beds; boulders larger than 2 feet across indicate they were deposited during flash flood events
• Granitic and gneissic cobble to boulder-sized clasts do not match the bedrock of the adjacent and upstream San Gabriel Mountains, nor do the smaller grains of porphyritic andesite… How did it get here??? 
• The fact that the lithology of the sediment within the Punchbowl Formation does not match the lithology of the local source area (San Gabriel Mountains) indicates this formation is exotic to this area and was therefore moved here from its place of formation, its depositional environment, by lateral movement along the SAF Zone
• Clasts from the Sidewinder Volcanic field, nearly 60 miles to the northeast, were transported across the SAF, deposited on the Pacific Plate, and delivered to the Devil’s Punchbowl via the SAF
• The Punchbowl Formation (Tp) is situated on the Pacific Plate, between the SAF to the north and the Punchbowl Fault to the south, causing squeezing of the strata by compressive stresses into the syncline known as the Devil’s Punchbowl

Cross-section of Syncline at Devil's Punchbowl; Tp = Tertiary Punchbowl Formation. –Dibblee, 1987.

Student activity 6: Ask, “What type of rock (igneous, sedimentary, metamorphic) is the Punchbowl Formation?” “Has the structure of the strata changed from its original position?” “Does this agree with the Law of Original Horizontality?”

Student activity 7: Instruct students to show the orientation or dip of the strata to their left (north) vs. to their right (south) using an outstretched hand. Ask students to discuss why there is a difference.

Leaving the Punchbowl Overlook, you’ll walk down the trail to the right (immediately to the north of the overlook) and begin the “Loop Trail” (not the Pinyon Pathway trail). Walk down the Loop Trail a few hundred feet to the first hairpin right turn, step over the dead tree, and walk past the “Trail à” signpost. Your next stop is the outcrops of the Punchbowl Formation dipping to the west just in front of you.

The first, smallest outcrop immediately left of the pinyon pine about 10 feet north of Loop Trail contains a large, nearly black clast from the Sidewinder volcanic field. Continue north (away from the Loop Trail) to the taller, more prominent, and instructive outcrops, but be careful to avoid the top of the cliff to your right. Alternatively, you could walk back towards the Loop Trail, turning right (west) just before reaching the Loop Trail to access a roughly-defined trail that takes you down and past the first small outcrop, along a wash, and about 100 feet to the dip slope side of the tall, prominent outcrops (see the photograph of outcrops).

Outcrops of the Punchbowl Formation. 

Student activity 8: Ask students to inspect Punchbowl Formation closely, then make a sketch of the Cajon Formation – like the sketch of the Cajon beds at Mormon Rock. Once they’ve finished, ask them if this rock is the same as the Cajon Formation at Mormon Rocks.

For “yes” answers, ask: “What makes you think they are the same?” And, “How did these rocks end up here?”

For the correct answer, “no” ask: “How do we know they are not the same?” Evidence for the answer to this question lies in the grains that make up some pebbles within the strata. In particular, the black to reddish clasts are volcanic sediments from the Sidewinder Volcanic field in the Mojave Desert, northeast of Victorville, about 60 miles away. Also, the other grains in the Punchbowl Formation do not match the rock that makes up the San Gabriel Mountains next to us. This means this sediment was not originally deposited here but must have been moved to this location. “How could this happen?”

After discussing and studying outcrops, you may wish to complete the Loop Trail. This moderately strenuous hike takes one down through outcrops of the Punchbowl formation, which provide opportunities for further study, to a small creek at the bottom of Devil’s Punchbowl, then back out again. Allow about an hour to complete. NOTE: If you choose to hike the entire loop trail, you should be prepared that some students may struggle walking back out of the punchbowl; on a few occasions, I’ve been sincerely worried about the health of some students. As a result, before descending to the bottom, I announce it’s a strenuous hike and optional. It’s perfectly okay for them to wait in the visitor’s center or immediately around the picnic area for the group to return.

Examine the stream channel at the bottom of the punchbowl with enough time. There, you can discuss stream channels – an excellent example of a “bedrock channel” – and discuss the lithology of the cobbles and boulders within the channel. 

Student activity 9: Ask students to identify the lithology of the cobbles and boulders within the channel.

Point out to students that the stream sediment, alluvium, has been eroded from the San Gabriel Mountains upstream and is different than the grains within the Punchbowl Formation. 

Point out some of the rock slides that are happening just above the channel. Return to the trail and hike to your left to complete the Loop Trail. 

Load up the vehicles and head home. I’d recommend checking Google Maps for the fastest route back. Depending on your origin and traffic, the best route could be west on CA-138 to CA-14 south or CA-138 east, back to the I-15 south.

Follow-up Questions

1. Explain how the shape of Cajon Creek and the juxtaposition of rock units near the Blue Cut area provide evidence for recent movement along the SAF.
2. What type of rock is Mormon Rocks? What is the specific name and age of the geologic formation? What was the depositional environment for the sediments comprising the Cajon beds at Mormon Rocks?
3. What is the geologic significance of the inface bluffs?
4. On what tectonic plate is Devil’s Punchbowl State Park found?
5. What type of rock comprises the Devil's Punchbowl? What is the specific name and age of the geologic formation? Under what geologic conditions were these sediments deposited?
6. How do we know that sediment in the Punchbowl Formation was deposited at a different geographic location than today?
7. Explain why the Cajon and Punchbowl formations are different rock units despite being very similar in appearance.
8. What type of geologic structure gives Devil’s Punchbowl its name? How does such a structure form?